(1) Field of the Invention
This invention relates to a color image exposure apparatus for forming a color positive from a color original by using a copying color-sensitive material which is generally employed for a color photograph printer, color copier or the like, especially, a copying color-sensitive material making use of a silver halide (hereinafter called "color paper").
(2) Description of the Related Art
For the control of exposure in a color copying machine, it is important to maintain in conformity the spectral sensitivity distribution of a light measuring system and that of an exposing system.
As apparatuses having the same spectral sensitivity distribution in both light measuring system and exposing system, there have been known color copying machines in which the spectral sensitivity distribution of a photoelectric transducer is made close to that of a copying sensitive material owing to the use of CdS as the photoelectric transducer in combination with a filter. A color photograph printer has also been proposed in which a trimming filter is inserted in an optical path between a light source and a photometer. The trimming filter permits transmission of rays of a high-sensitivity wavelength among the individual photosensitive ranges of blue(B), green(G) and red(R) sensitive layers of a color paper and cuts off rays of a low-sensitivity wavelength among the individual photosensitive ranges (Japanese Patent Application Laid-Open No. 64037/1978).
A filter, which cuts off rays in a particular wavelength band by either absorbing or reflecting them and permits transmission of rays of a wavelength shorter or longer than the wavelength band as described above, will hereinafter be called a band-stop filter (due to characteristics opposite to a band-pass filter).
However, the position suitable for the arrangement of such a filter, for example, a filter provided with a dielectric multilayer film is said to be any position between the light source and the color paper in the technique of Japanese Patent Application Laid-Open No. 64037/1978 referred to above. This arrangement however involves the potential problem that some characteristics of the filter provided with the dielectric multilayer film may be impaired by a wavelength shift caused by heat or at a certain incident angle. In this respect, similar inconvenience is also encountered in the technique disclosed in Japanese Patent Application Laid-Open No. 113627/1976. Although the spectral distribution of a filter provided with a dielectric multilayer film varies depending on the position of its arrangement, this variation may be divided into two aspects, one permitting its compensation by predicting the degree of the variation in advance and designing the characteristics of a filter with the prediction in mind and the other hardly permitting such a compensation. It is indispensable to obtain precise spectral characteristics for drawing out suitable effects of a filter provided with a dielectric multilayer film. An appropriate optical system including a filter provided with a dielectric multilayer film is required for this purpose.
FIG. 2 diagrammatically illustrates results of an experiment on the relative energy distribution of color light around a color paper when a color-light limiting filter coated with a dielectric multilayer film is arranged at a predetermined point between a light source and the color paper. The diagram is drawn on an enlarged scale to show the relative energy distribution around 500 nm.
Curve A in FIG. 2 is a characteristic curve obtained when a color photograph printer was constructed without a diaphragm mechanism 40 of FIG. 4 and a BG limiting filter a and an infrared cutoff filter d were interposed between a lamp house 20 and a light diffuser box 16 to provide a dielectric multilayer film having characteristics of FIG. 15. As will be appreciated from a comparison between the characteristic diagram of FIG. 15 and curve A of FIG. 2, a wavelength shift of about 20 nm took place toward the shorter wavelength side in the above case. Further, the difference in transmittance between the reflected band and the transmitted band was reduced so that the effects of the BG limiting filter a were reduced significantly. As a result, coupled with the wavelength shift toward the shorter wavelength side, the sensitivity of the blue sensitive layer was lowered.
In the trimming filter disclosed in Japanese Patent Application Laid-Open No. 64037/1978, the improved color reproducibility has been obtained as a secondary effect by cutting off both ends of the spectral sensitivity distribution of a color paper. However, spectral characteristics of each pigment in a color paper are not taken into consideration at all. In addition, the color reproducibility cannot be improved under such conditions that cutoff of spectral rays is conducted to such an extent as to not cause any drop in the second sensitivity peak on the side of wavelengths longer than the inherent sensitivity peak of the blue sensitivity. It is hence difficult to simultaneously realize both effects, namely, conformance of spectral sensitivity distribution and improvements in color reproducibility under conditions where the sensitivity reduction is small.
FIG. 3 illustrates average density distributions of pigments of the three primary colors of Y(yellow), M(magenta) and C(cyan) when the mask density of a color negative film has been removed. As shown in FIG. 3, the spectral distributions of the individual pigments overlap. It has been known that the color reproducibility of a color print deteriorates as the densities of the overlapped parts become higher. For confirmation of this, film pigments C,M,Y may be represented in terms of printing density ratios (pigment color mixing ratios) of R, G and B respectively as shown in Table 1.
TABLE 1 ______________________________________ Pigment Printing density C M Y ______________________________________ R density (Dr) 1.00 0.09 0.00 G density (Dg) 0.06 1.00 0.23 B density (Db) 0.00 0.11 1.00 ______________________________________
Here, the printing densities (Dr,Dg,Db) of the respective colors can be obtained in accordance with the following formula: ##EQU1## where J.lambda.: Spectral luminance distribution
Si.lambda.: Relative sensitivity distribution of a photosensitive layer i of a color paper PA1 T.sub.j .lambda.: Spectral transmittance distribution of pigment j of a color film PA1 i: One of red, green and blue PA1 j: One of cyan, magenta and yellow
As is understood from Table 1, the printing density of a green sensitive layer for the yellow pigment is 0.23 while the printing density of the blue sensitive layer for a magenta pigment is 0.11. These printing densities are much higher compared to the printing density for the magenta pigment itself (R printing density: 0.09) or that for a cyan pigment (G printing density: 0.06). This can be attributed primarily to the inclusion of a high secondary photosensitive range at 490-520 nm of the green sensitive layer. The M band of a G sensitization pigment is absorbed in the secondary photosensitive range. This secondary photosensitive range is a photosensitive range which exists on the wavelength side shorter that that corresponding to the maximum sensitivity. Moreover, the use of a filter provided with a dielectric multilayer film is indispensable to practise the conventional technique. In such a case, unless the heat of the light source and the incident angle of light to the filter are suitably controlled, desired characteristics cannot be obtained, and when they are not appropriate, the use of such a filter may result in deleterious effects on the contrary.